Interferometer for testing large surfaces



July 10, 1962 J. B. SAUNDERS INTERFEROMETER FOR TESTING LARGE SURFACES 2Sheets-Sheet 1 Filed March 21, 1960 823%5 5 am Q a m H M Q E H M w m 5QB 5 M @5828 w 33% m I Q 0 O w ak M w o9 Q Q 3 3 5 Q X m 3 6 6% H 1 S JI Q: Q

July 10, 1962 J. B. SAUNDERS 3,043,132

INTERFEROMETER FOR TESTING LARGE SURFACES Filed March 21, 1960 2Sheets-Sheet Fly 24 my 25 INVENT OR fame; B. Jaundens ATTORNEYS UnitedStates 3,043,182 INTERFEROIAEIER FOR TESTING LARGE SURFACES James B.Saunders, Alexandria, Va., assignor to the United States of America asrepresented by the Secretary of Commerce Filed Mar. 21, 1960, Ser. No.16,617 3 Claims. (Cl. 88-14) The present invention relates to aninterferometer that permits the testing of precision flat surfaces andmore particularly to a simple, sensitive interferometer for meastiringthe flatness of large surfaces, i.e., up to 16 feet and more in length.

Such an instrument greatly facilitates the testing of precision flatsurfaces such as aircraft fixtures, machine ways and layout plates whichmust be carefully examined for irregularities. The subjectinterferometer not only allows a quick check on surface quality but alsoprovides a means of production control by locating the spots where addedpolishing or lapping are required.

The only other known interferometric device designed for investigatinglarge areas has not heretofore been used extensively due primarily toits extreme complexity. The instrument of the subject invention, on theother hand, is relatively simple and easy to operate, is twice assensitive as the referred-to prior art interferometer and is relativelyfree from vibration, thereby providing an interferometric device whichaids materially in the control of dimensional tolerances ofmachine-produced parts.

It is accordingly an immediate object of the present invention toprovide an improved interferometer enabling the rapid determination offlatness in connection with large surfaces without requiring contactwith the surface being measured.

It is a further object of this invention to provide such a device whichis easy to use and maintain in good working condition and which isadaptable to a variety of alignment testing problems.

Another object of the invention is to provide a device by which a singleoperator may quickly obtain very precise angular measurements of thealignment of extended surfaces.

Still another object of this invention is to provide a device whichallows closer dimensional control of machine-produced. parts.

Other objects and features of the invention will become apparent tothose skilled in the art as the disclosure is made in the followingdetailed description of a preferred embodiment of the invention asillustrated in the accompanying sheets of drawings in which:

FIG. 1 is a symbolic drawing of an interferometric alignment apparatusillustrating the optical principles of the invention;

FIGS. 2A-2D show typical fringe patterns produced,

by the apparatus of FIG. 1 wherein the angle of incidence ice As inother interferometers, the basic principle utilized in the presentinvention is that if two beams of light from the same source travelalong difierent paths and then come together again they will form apattern of interference fringes, the number of fringes is dependent uponthe difference in optical path length.

The interferometer of this invention permits extension to large areas bycausing a collimated beam of light to reflect from the specimen at alarge angle of incidence.

of light on the surface plate undergoing test is approximately 87 /2FIG. 2A illustrates such a fringe pattern produced by white light on alayout plate having a granite surface;

FIG. 2B illustrates a pattern produced by monochromatic light on alayout plate having a badly scratched cast iron surface; 7

FIGS. 2C-2D illustrate fringe patterns produced by monochromatic lighton the granite surface of FIG. 2A;

FIG. 3 is a diagrammatic representation showing a modified Ktistersdouble-image prism to be utilized with monochromatic light; and

FIG-4 is a graphic& presentation of the fringe pattern of FIG. 2Awherein the surface contour in ,u inches is plotted against the distancealong the surface plate in inches.

The resultant fringe pattern is a contour map of the surface relative toan arbitrarily chosen plane and the contour interval is a function'ofwavelength and angle of incidence. Each pattern is formed by theinterference of light rays reflecting from different parts of the samesurface, a result obtained by using a double-image prism. Areas of anysize can be tested by merely adjusting the angle of incidence of thelight on the surface, the width of the area examined at each settingbeing equal to the linear aperture of the instrument.

The principal optical components employed in the invention arediagrammatically shown in FIG. 1. A double-image Kosters prism 11 ismade by taking two 306090 prisms, HA-41B, respectively, depositing asemitransparent layer of aluminum on a face of one of them and cementingthe two together with the aluminized face on the inside. The resultingunit is a 6060 60 prism, the partially aluminized surface of whichbecomes a semireflecting plane. The detailed construction of such aprism is described in an article by the present applicant in the NBSJournal of Research (January 1957) For purposes of convenience only,henceforth throughout the body of the specification and the claims, thealuminized layer heretofore referred to will be designated as thecentral or dividing plane.

With reference to FIG. 1 of the drawings, the interferometer 10 of thisinvention includes a Kosters doubleimage prism indicated generally at 11and a reflecting or mirror unit indicated generally at 12. Light from apoint source 13 is collimated into a parallel beam of light by a lens14, the axis of which is approximately normal to a face 11a of thedouble-image prism 11. A viewing tube (not shown) containing anobjective lens 15 has its axis approximately normal to face 11b ofdouble-image prism 11.

The principal element of the interferometer 1G is the double-imageKosters prism 11, constructed in the manner heretofore described, whichis mounted in any conventional manner so as to place the dividing planeof said prism at a desired angle with respect to the sur: face to betested, however, it is to be understood that said dividing plane may beadjustably positioned at any angle with respect to the test surface aswill hereinafter he discussed in detail.

Referring again to FIG. 1 of the drawings, wherein the light ray pathsof the interferometer 10 are diagrammatically illustrated, the opticalaxis of the collimating lens 14 is substantially perpendicular to face11a so as to produce oblique incidence of the beam upon the dividingplane 11c. The beam of light passed by lens 14 is separated into twocoherent beams 11a and 11b by'dividing continues ahead to the oppositeface 11b of the doubleconventional manner.

image prism 11 and is then internally reflected outward. The other ray,11:1 is bent back by the dividing plane 110 to the face 1111 throughwhich it entered and then reflected outward as indicated by the arrows(see FIG. 1). The two emerging rays (which derive from a single originalray) are symmetrical with respect to the central plane. In a mannerhereinafter to be described, the entrance angle of the collimated lightmay be adjusted to a desired deviation between the component rays Ita-1112 As diagrammatically shown in FIG. 1 of the drawings, rays 11a 11bare directed to impinge at normal incidence onto distant plane mirrors Mand M respective ly, which are held in substantially vertical positionin a The reflectivity of mirrors M and M are designed to giveapproximatvely equal inensity for the two light beams at a receivingpoint 16.

When the positions of mirrors M -M are properly adjusted/the beams arereflected back along their outgoing paths into the dividing plane 110where the beams recombine in interfering relation. For example, the beamof light of which 1 1x1 is typical is reflected from a test surface 17at an angle of incidence of degrees (5 is the angle of incidence oflight), then normally from mirror M and returns upon itself to combinewith ray llb at the dividing plane 110. The observer at 16 secsinterference fringes on the superimposed images of the two mirrors,which interference fringes are a measure of surface evenness andflatness. the angle of incidence 3 surfaces of any length can be tested.

In general, the fringes that appear at 16 represent contours of equalelevation. They lie along straight lines if the surface under test isperfectly flat, but are curved if the surface is not flat. Smallirregularities which do not affect over-all flatness characteristicsproduce serrated or ragged fringes. The number of fringes and thereforethe fringe width'is adjustable but the contour intervalthereby'represented depends upon the Wavelength of the light used andthe angle of incidence.

Specifically, the gross aspects of the fringe pattern a (fringedirection and spacing) are controlled by the wedge angle 0 between thewave fronts of beams Ha l lb while the small irregularities of thefringe pattern are a function of the irregularities of the'test surface.If the test surface 17 is perfectly flat, the fringes will be straightand parallel. Any curvature of the test surface introduces acorresponding curvature in the wave front of beam Ha and this wavefront, when compared with the wave front of 'wave 1112 in a mannerheretofore described, introduces curvature into the otherwise straightfringes. V

Referring now to FIGS. 2A-2D of the drawings, typical interferencefringe patterns produced by the large surface interferometer. of FIG. 1are illustrated. These patterns represent an area 36 inches in length by1% inches in width wherein'the angle of incidence of light upon the testsurface is approximately 87 /2 degrees. Because of the large angle ofincidence; the test beam of 1% inch diameter can illuminate the fulllength of a test surface whose length is more than twenty times thediameter of the test beam.

Futhermore, by adjusting.

The pattern of FIG. 2A illustrates a pattern produced I with white lightupon a granite surface layout plateaud shows the concavity of thesurface. Since the reflectivity of a surface increases with angles ofincidence, this relatively rough (diffusely reflective) surface wastested'with light having a large angle i.e., slightly less than ninetydegrees) of incidence. "As shown in the fringe pattern,

very smooth fringes were obtained, which demonstrates that the roughsurface reflected theincident test beam in d a substantially regularmanner.

d The patterns of FIGS. 2B2D were" produced monochromatic,( \=5876 A.)light. FIG. 2B illustrates a a pattern produced by monochromatic lightupon a badly a 4 scratched (diifusely reflective) cast iron surface. Insaid pattern a departure from straightness of one fringe corresponds toa deviation from flatness of 5.75% (approximately 0.00013 inch).Although this surface was badly scratched, the test results show thatits over-all flatness characteristics are excellent.

FIGS. 2C-2D illustrate patterns produced by monochromatic light upon thegranite surface utilized in FIG. 2A. The difierence between patterns2C-2D results from changing the angle between the two interferingwavefronts, thereby changing the width of the fringes.

As described in connection with the fringe pattern of FIG. 2A, whitelight may be used, however, the resulting difficulty in finding andadjusting the fringes exceeds the inconveniences associated withmonochromatic light. If, however, white light is to be used,differential reaction may be reduced to a minimum by using the modifiedKosters prism shown in FIG. 3 of the drawings. The procedure for makingsuch a prism differs from that heretofore described in that all surfacesare finished before cutting the prism into two parts.

There is shown in FIG. 4 of the drawings data plotted from theinterferometric measurements on the granite surface layout plate of FIG.2A In said figure the surface contour in ,u inches is plotted againstthe distance along the surface plate in inches. Irregularities as smallas 10a inches are determinable from said graph.

In the prior art interferometers previously used for measuring flatness,a one-fringe departure from straightness corresponds to a surfacedeparture from flatness of A (2 cos B) where A is the wavelength oflight and ,8 the angle of incidence. In the present invention, however,wherein the sensitivity of the instrument depends upon the value of ,8,since light is reflected twice from the test surface the sensitivity isdouble that of conventional interferometers, that is, equal to 7\ (4 cos,8) for the same angle of incidence. Consequently, even though thesensitivity is lowered by increasing the angle of incidence to coverlong surfaces, it is sufliciently high for current standardsrequirements. V

To test this instrument, a l%-inch aperturesystern was adjusted to covera 36-inch surface, requiring an 87 /2 angle of incidence. 0.5878 forexample, one fringe corresponds to approximately 0.0001 inch. As anexperienced observer can estimate fringes to better than one-tenth of afringe spacing, surface irregularities of approximately lo -inch arereadily determinable. p a

The maximum length of surface that may be covered with one setting is Asec 5, where A is-the aperture of the prism. Thus, by changing 5, anylength is covered with a prism of a given aperture, it is understood, ofcourse, that the sensitivity of the'instrument decreases with increasingangles of incidence.

- In a modification of the instrumentof' this invention, apparatus suchas is Well known in the art is employed to provide variable angles 0and/3. In this manner a test surface of any desired length can be madeto fill the aperture of the prism, thereby producing maximum sensitivityfor. the surface measured. 1 However, since the value of ,6 must-beknown for evaluating a fringe pat tern, inanother'modification of theinventiomfixed values of 0 and b are utilized with adjustableen'd'm'irrors M andM said mirrors are bound into a rigid unit and themirror unit and prism housing also rigidly bound together, therebyeliminating the necessity of making frequent measurement of 9 and alsoadding to the stability of the instrument. Such an'instrument has afixed adjustment and can be used by an unskilled operator.

Alternatively, a cover (not shown) that encloses the light beams andpositioned between the prism and end mirrors, respectively, greatlyenhances the stability of is inasteady state.

Itis understood, of course, that the foregoing disthe fringes and is tobe used'unless the surrounding air With light having a wavelength ofclosure relates to only a preferred embodiment of the invention and thatit is intended to cover all changes and modifications of the examples ofthe invention herein chosen for the purposes of the disclosure, which donot constitute departures from the spirit and scope of the invention.

What is claimed is:

1. In an interferometer wherein a collimated beam of light is dividedinto reference and test beams'which are reflected from a piano referencemirror and a test surface, respectively, and recombined so as to forminterference fringes which are indicative of the flatness of the testsurface, the combination of: a diffusely reflective test surface Whoselength is many times greater than the diameter of said test beamdisposed relative to the axis of said test beam at an angle of incidenceof slightly less than 90", whereby substantially the entire length ofsaid test surface is illuminated by said test beam and said test beam isreflected from said test surface in a substan tially regular manner, andan auxiliary plano mirror disposed normal to the axis of the beamreflected from said test surface so as to cause said reflected beam toretrace its path.

2. Interferometric apparatus for testing the flatness of large surfaces,comprising: means for projecting a collimated beam of light, means fordividing said collimated beam into reference and test beams, planoreference mirror means for reflecting said reference beam back along itsoriginal path to said dividing means, means for holding a test surfacewhose length is many times greater than the diameter of said test beamrelative to the axis of said test beam atsuch an angle of incidence thatsubstantially the entire length of said test surface is illuminated bysaid test beam, auxiliary plano mirror means disposed on the far side ofsaid test surface holding means for reflecting said test beam back alongits original path to said dividing means, said dividing means comprisingmeans for combining said reflected reference and test beams so as toproduce interference fringes, and means for viewing said fringes.

3. Apparatus as set forth in claim 2 wherein said angle of incidence ofsaid test beam on said test surface is slightly less than whereby saidtest surface may be diffusely reflective.

References Cited in the file of this patent UNITED STATES PATENTS1,824,668 Hasselkus et al. Sept. 22, 1931 2,880,644 Brockway et al Apr.7, 1959 FOREIGN PATENTS 646,760 Great Britain Nov. 29, 1950 OTHERREFERENCES Article, Interferometer Tests Large Surfaces, Instruments andControl Systems, vol. 32, page 634, May 1959.

